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Chapter 23: The Transport System

23.1 Open and Closed Circulatory Systems

  • The circulatory system:

    • Delivers oxygen and nutrients to cells.

    • Removes carbon dioxide and waste materials.

    • Some animals don't need a circulatory system.

    • Hydra: cells are part of a single layer of external cells or line the gastrovascular cavity.

    • Each cell is exposed to water and can independently exchange gases and rid itself of wastes.

    • Planarian: digestive cavity branches throughout the small, flattened body.

    • No cell is very far from one of the digestive branches, so nutrient molecules can diffuse from cell to cell.

    • Diffusion meets the respiratory and excretory needs of the cells.

    • Some animals rely on the movement of fluid within a body cavity (or coelom) to circulate gases, nutrients, and wastes, such as nematodes and echinoderms.

Animals without circulatory systems.

Open Circulatory Systems

  • A circulatory system consists of a heart and associated vessels.

  • The heart's role is to keep the fluid (blood) moving within the vessels.

  • Circulatory systems may be classified as open or closed.

  • The grasshopper has an open circulatory system.

  • In an open circulatory system, fluid is not always confined to the vessels.

  • A tubular heart pumps hemolymph through a network of channels and cavities in the body.

  • The cavities, or sinuses, contain the animal's organs and are collectively known as the hemocoel.

  • Hemolymph drains back to the heart.

  • The hemolymph of a grasshopper is colorless and does not contain hemoglobin or any other respiratory pigment.

  • Oxygen is taken to cells, and carbon dioxide is removed from them by way of air tubes called tracheae.

  • Tracheae are found throughout the body and provide efficient transport and delivery of respiratory gases while restricting water loss.

Open circulatory system.

Closed Circulatory System

  • All vertebrates and some invertebrates have a closed circulatory system, also known as a cardiovascular system.

  • The system consists of a muscular heart and blood vessels.

  • Blood remains within the blood vessels at all times.

  • Humans have two receiving chambers (atria) and two pumping chambers (ventricles) in the heart.

  • There are three types of vessels: arteries, capillaries, and veins.

  • Arteries carry blood away from the heart, capillaries exchange materials with interstitial fluid, and veins return blood to the heart.

  • Blood is always contained within these vessels and does not run freely into the body unless there is an injury.

  • As blood passes through capillaries, some water is forced out and into the interstitial fluid.

  • Some of this fluid returns directly to a capillary, while some is picked up by lymphatic capillaries.

  • The fluid, now called lymph, is returned to the cardiovascular system by lymphatic vessels.

Closed circulatory systems.

Comparison of Vertebrate Circulatory Pathways

  • Two types of circulatory pathways among vertebrate animals:

    • Fishes have a one-circuit (single-loop) pathway through the body

      • The heart has a single atrium and a single ventricle.

      • The ventricle pumps blood to the gills for gas exchange.

      • Gill capillaries receive O2-poor blood; systemic capillaries receive O2-rich blood.

    • Other vertebrates have a two-circuit (double-loop) circulatory pathway

      • The heart pumps blood to tissues through a systemic circuit.

      • The heart pumps blood to the lungs through the pulmonary circuit.

      • The double pumping action is seen in terrestrial animals that breathe air.

Comparison of circulatory circuits in vertebrates.

  • Amphibians have two atria and a single ventricle.

  • Some mixing of O2-rich and O2-poor blood occurs in amphibians.

  • Most reptiles have a single ventricle with a partial septum, reducing mixing of blood.

  • Crocodilians, birds, and mammals have hearts divided into right and left halves.

  • The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body.

  • The left ventricle is larger than the right ventricle.

  • This arrangement provides adequate blood pressure for both the pulmonary and systemic circuits.

23.2 Transport in Humans

  • The human cardiovascular system pumps blood into blood vessels.

  • Blood vessels take blood to capillaries.

  • Exchanges take place in capillaries.

  • In the lungs, carbon dioxide is exchanged for oxygen.

  • In the tissues, nutrients and oxygen are exchanged for carbon dioxide and other wastes.

  • If the heart stops pumping, death results because exchanges in the lungs and tissues are crucial.

The Human Heart

  • The right side pumps O2-poor blood to the lungs.

  • The left side pumps O2-rich blood to the tissues.

  • The septum separates the right side from the left and prevents the mixing of blood.

  • Each side has two chambers: atria (upper, thin-walled) and ventricles (lower, thick-walled.)

  • Valves are located between the atria and ventricles and between the ventricles and attached vessels.

  • Valves close after blood moves through to keep it moving in the correct direction.

  • Atrioventricular valves are between atria and ventricles, and semilunar valves are between ventricles and attached vessels.

  • Blood path: right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk and arteries → lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → tissues.

  • Pulmonary arteries carry O2-poor blood, and pulmonary veins carry O2-rich blood (opposite of other arteries and veins.)

  • Arteries take blood away from the heart; veins take blood toward the heart.

Heart anatomy and the path of blood throughthe heart.

The Cardiac Cycle

  • The heart's pumping action is called the cardiac cycle.

  • The cardiac cycle consists of the following events:

    • Atrial systole: The atria contract, pumping blood into the ventricles.

    • Ventricular systole: The ventricles contract, pumping blood out of the heart.

    • Diastole: The atria and ventricles relax, filling with blood.

  • The cardiac cycle is controlled by the SA node, a group of cells in the heart that generate electrical impulses.

  • The SA node sends out an electrical impulse every 0.85 seconds, which causes the heart to beat.

  • The heart rate can be affected by factors such as exercise, stress, and medications.

  • An electrocardiogram (ECG) is a test that can be used to measure the electrical activity of the heart.

  • ECGs can be used to diagnose heart problems such as arrhythmias and heart attacks.

  • A defibrillator is a device that can be used to shock the heart back into a normal rhythm.

The cardiac cycle.

Control of the heartbeat.

Blood Vessels

  • Arteries:

    • Transport blood away from the heart.

    • Have a thick wall with a middle layer of smooth muscle and elastic fibers.

    • Elastic fibers allow arteries to expand and accommodate sudden increase in blood volume.

    • Smooth muscle strengthens the wall and prevents overexpansion.

    • Branch into arterioles, which are small arteries that can be regulated by the nervous system.

    • Arterioles can be dilated or constricted to control blood flow and raise blood pressure.

    • Arterioles branch into capillaries, which are extremely narrow tubes that allow exchange of nutrients and waste molecules.

    • Capillary beds are widespread and allow efficient exchange of substances between the blood and interstitial fluid.

    • The entrance to capillary bed is controlled by precapillary sphincters.

    • Blood moves through a shunt from arteriole to venule if capillary bed is closed.

  • Veins:

    • Collect blood from capillary beds and carry it back to the heart.

    • Have thinner walls and wider diameters than arteries.

    • Skeletal muscle contraction pushes blood past valves that prevent backflow.

    • Valves point toward the heart.

    • Blood flows in the direction of reduced pressure, aided by inhalation and chest expansion.

Blood vessels.

Movement of blood in a vein.

The Pulmonary and Systemic Circuits

  • The human cardiovascular system has two major circulatory pathways:

    • Pulmonary circuit

      • Moves blood to and from the lungs

      • Converts O2-poor blood to O2-rich blood

    • Systemic circuit

      • Moves blood to and from other tissues of the body

      • Serves the metabolic needs of the body's cells

  • Figure 23.10 shows the path of blood in both circuits.

23.10 The path of blood.

  • The circulatory system can be divided into two circuits: the pulmonary circuit and the systemic circuit.

  • Pulmonary Circuit:

    • O2-poor blood from all regions of the body collects in the right atrium.

    • Blood passes into the right ventricle.

    • The right ventricle pumps blood to the lungs via the pulmonary trunk and the pulmonary arteries.

    • Carbon dioxide is given off and oxygen is picked up as blood passes through pulmonary capillaries.

    • O2-rich blood returns to the heart via the pulmonary veins.

    • The pulmonary veins enter the left atrium.

  • Systemic Circuit:

    • O2-rich blood enters the left atrium from the lungs and passes into the left ventricle.

    • The left ventricle pumps the blood into the aorta.

    • Arteries branching from the aorta carry blood to all areas and organs of the body.

    • Blood passes through capillaries and collects in veins.

    • Veins converge on the venae cavae, which return the O2-poor blood to the right atrium.

    • Arteries contain O2-rich blood and are bright red; veins contain O2-poor blood and appear dull red or blue when viewed through the skin.

  • Portal System:

    • A portal system begins and ends in capillaries.

    • The hepatic portal vein takes blood from the intestines to the liver.

    • The liver modifies substances absorbed by the intestines and monitors the normal composition of the blood.

    • The hepatic veins carry blood out of the liver into the inferior vena cava.

Lymphatic System

  • Consists of lymphatic vessels and various lymphatic organs.

  • Takes up fat in the form of lipoproteins from the digestive tract and transports it to the circulatory system.

  • Works with the immune system to help defend the body against disease.

  • Takes up excess interstitial fluid and returns it to cardiovascular veins in the shoulders.

  • Most regions of the body are richly supplied with lymphatic capillaries.

  • The construction of the larger lymphatic vessels is similar to that of cardiovascular veins, including the presence of valves.

  • The movement of lymph within these vessels is dependent on skeletal muscle contraction.

  • The lymphatic system is a one-way system that begins at the lymphatic capillaries.

  • The lymphatic capillaries take up excess interstitial fluid, and once it enters the lymphatic vessels, it is called lymph.

  • The lymphatic capillaries join to form larger lymphatic vessels that merge before entering one of two ducts: the thoracic duct or the right lymphatic duct.

  • The thoracic duct is much larger than the right lymphatic duct and serves the lower limbs, abdomen, left arm, and left sides of both the head and the neck.

  • The right lymphatic duct serves the right arm, the right sides of both the head and the neck, and the right thoracic area.

  • The lymphatic ducts enter the subclavian veins.

Lymphatic vessels.

Capillary Exchange in the Tissues

  • Exchange in animals:

    • In some animals, exchange is carried out by each cell individually because there is no cardiovascular system.

    • In animals with a cardiovascular system, interstitial fluid is involved in exchanging materials between capillaries and cells.

    • Amino acids, oxygen, and glucose exit a capillary and enter interstitial fluid to be used by cells.

    • Carbon dioxide and wastes leave interstitial fluid and enter a capillary to be excreted from the body.

    • The cardiovascular system takes blood to the capillaries where exchange occurs.

    • Homeostasis is not maintained without this exchange, and cells perish.

  • Mechanics of capillary exchange:

    • Blood pressure and osmotic pressure are two opposing forces at work along the length of a capillary.

    • Blood pressure is caused by the beating of the heart, while osmotic pressure is due to the salt and protein content of the blood.

    • Blood pressure holds sway at the arterial end of a capillary, and water exits.

    • Blood pressure is reduced by the time blood reaches the venous end of a capillary, and osmotic pressure now causes water to enter.

    • Midway between the arterial and venous ends of a capillary, blood pressure pretty much equals osmotic pressure, and passive diffusion alone causes nutrients to exit and wastes to enter.

    • Diffusion works because interstitial fluid always contains fewer nutrients and more waste than blood does.

  • Lymphatic system:

    • The exchange of water at a capillary is not exact, and excess interstitial fluid is always produced.

    • Excess interstitial fluid is collected by lymphatic capillaries and becomes lymph.

    • Lymph contains all the components of plasma, except it has much less protein.

    • Lymph is returned to the cardiovascular system when the major lymphatic vessels enter the subclavian veins in the shoulder region.

  • Distribution of heat:

    • The blood distributes heat to body parts.

    • When warm, capillaries that serve the skin are open, and the face is flushed.

    • This helps rid the body of excess heat.

    • When cold, skin capillaries close, conserving heat.

Capillary exchange.

23.3 Blood: A Transport Medium

  • Blood is a form of fluid connective tissue

  • Blood has important roles in the body, including:

    • Transport of substances to and from the capillaries

    • Defense of the body against invasion by pathogens

    • Assisting in homeostasis by regulating body temperature and pH

  • Blood in humans has two main portions:

    • Liquid portion called the plasma.

    • Formed elements consisting of various cells and platelets.

  • These portions can be separated by spinning blood in a centrifuge (Fig. 23.14).

Fig 23.14 Components of blood.

Plasma

  • Plasma is mostly composed of water (90-92%) and proteins (7-8%).

  • Plasma also contains smaller quantities of nutrients, wastes, and salts.

  • The salts and proteins in plasma help buffer the blood and maintain a slightly basic pH of 7.4.

  • They also maintain the blood's osmotic pressure, allowing water to enter blood capillaries.

  • Several plasma proteins are involved in blood clotting.

  • Other plasma proteins transport large organic molecules in the blood.

  • Albumin, the most abundant plasma protein, transports bilirubin, a breakdown product of hemoglobin.

  • Lipoproteins transport cholesterol.

Formed Elements

  • The formed elements in blood are red blood cells, white blood cells, and platelets.

  • Red blood cells, also known as erythrocytes, transport oxygen.

  • Red blood cells lack a nucleus and contain hemoglobin, a respiratory pigment.

  • There are 6 million red blood cells in a small drop of whole blood, each containing about 250 million hemoglobin molecules.

  • Hemoglobin contains iron, which loosely combines with oxygen to transport it.

  • Insufficient red blood cells or hemoglobin causes anemia and fatigue.

  • High altitudes stimulate red blood cell formation.

  • Red blood cells are continuously manufactured in certain bones, stimulated by the hormone erythropoietin (EPO).

  • EPO is produced by the kidneys and liver, and is available as a drug to treat anemia.

  • EPO has been abused by athletes to enhance performance.

  • Red blood cells synthesize hemoglobin and lose their nuclei before being released from the bone marrow into the blood.

  • After living for about 120 days, red blood cells are destroyed, mainly in the liver and spleen, where they are engulfed by large, phagocytic cells.

  • Hemoglobin is released when red blood cells are destroyed. The iron is recovered and returned to the red bone marrow for reuse.

  • Another portion of the molecules (i.e., heme) undergoes chemical degradation and is excreted by the liver as bile pigments in the bile. The bile pigments are primarily responsible for the color of feces.

  • White blood cells, also called leukocytes, help fight infections. They are usually larger than red blood cells, have a nucleus, lack hemoglobin, and appear translucent if unstained.

  • There are approximately 5,000–11,000 white blood cells in a small drop of whole blood.

  • Growth factors are available to increase the production of all white blood cells, and these are helpful to people with low immunity, such as AIDS patients.

  • Red blood cells are confined to the blood, but white blood cells are able to squeeze between the cells of a capillary wall. Therefore, they are found in interstitial fluid, lymph, and lymphatic organs.

  • When an infection is present, white blood cells greatly increase in number. Many white blood cells live only a few days, while others live months or even years.

  • Neutrophils are white blood cells that squeeze through the capillary wall and enter the interstitial fluid, where they phagocytize foreign material. Monocytes come on the scene next and are transformed into macrophages—large, phagocytizing cells that release white blood cell growth factors.

  • Lymphocytes, another type of white blood cell, play an important role in fighting infection. T cells attack the body’s cells that are infected with viruses, while B cells produce antibodies to protect the body against certain types of antigens.

  • An antigen is most often a protein but sometimes a polysaccharide. Antigens are present in the outer covering of parasites or in their toxins. When antibodies combine with antigens, the complex is often phagocytized by a macrophage.

Formed elements.

Platelets and Blood Clotting

  • Platelets are fragments of megakaryocytes in the red bone marrow

  • 200 billion platelets are produced daily

  • A small drop of whole blood contains 150,000 to 300,000 platelets

  • Platelets are involved in blood clotting or coagulation

  • Hemophilia is an inherited clotting disorder caused by the liver's inability to produce one of the clotting factors

  • Prothrombin and fibrinogen are two proteins involved in blood clotting that are manufactured and deposited in blood by the liver

  • Vitamin K is necessary for the production of prothrombin and can be found in green vegetables and formed by intestinal bacteria

  • Platelets clump at the site of a damaged blood vessel and form a plug that temporarily seals the leak

  • Prothrombin activator converts prothrombin to thrombin, which requires calcium ions

  • Thrombin severs two short amino acid chains from each fibrinogen molecule, forming long threads of fibrin that wind around the platelet plug and provide the framework for the clot

  • Red blood cells are trapped within the fibrin threads, making the clot appear red

  • Clot retraction follows, during which the clot gets smaller as platelets contract and serum is squeezed from the clot

  • A fibrin clot is present only temporarily, and plasmin destroys the fibrin network and restores the fluidity of the plasma as soon as blood vessel repair is initiated.

Blood clotting.

Cardiovascular Disorders

  • Cardiovascular disease (CVD) is the leading cause of untimely death in Western countries.

  • In the United States, about 31% of the population suffers from hypertension, which is high blood pressure.

  • Normal blood pressure is 120/80 mm Hg.

  • Hypertension occurs when blood pressure readings are higher than these numbers.

  • Hypertension is sometimes called a silent killer because it may not be detected until a stroke or heart attack occurs.

  • Heredity and lifestyle contribute to hypertension.

  • Hypertension is often seen in individuals who have atherosclerosis, an accumulation of soft masses of fatty materials, particularly cholesterol, beneath the inner linings of arteries.

  • Atherosclerosis begins in early adulthood and develops progressively through middle age.

  • To prevent its onset and development, a diet low in saturated fat and rich in fruits and vegetables is recommended.

  • Smoking, alcohol or other drug abuse, obesity, and lack of exercise contribute to the risk of atherosclerosis.

  • Plaque can cause a clot to form on the irregular arterial wall.

  • If thromboembolism is not treated, serious health problems can result.

  • A cerebrovascular accident, also called a stroke, often occurs when a small cranial arteriole bursts or is blocked by an embolus.

  • If a coronary artery becomes completely blocked due to thromboembolism, a heart attack can result.

  • Two surgical procedures are frequently performed to correct a blockage or facilitate blood flow: coronary bypass operation and balloon angioplasty.

Atherosclerosis.

Treatment for blocked coronary arteries.

I

Chapter 23: The Transport System

23.1 Open and Closed Circulatory Systems

  • The circulatory system:

    • Delivers oxygen and nutrients to cells.

    • Removes carbon dioxide and waste materials.

    • Some animals don't need a circulatory system.

    • Hydra: cells are part of a single layer of external cells or line the gastrovascular cavity.

    • Each cell is exposed to water and can independently exchange gases and rid itself of wastes.

    • Planarian: digestive cavity branches throughout the small, flattened body.

    • No cell is very far from one of the digestive branches, so nutrient molecules can diffuse from cell to cell.

    • Diffusion meets the respiratory and excretory needs of the cells.

    • Some animals rely on the movement of fluid within a body cavity (or coelom) to circulate gases, nutrients, and wastes, such as nematodes and echinoderms.

Animals without circulatory systems.

Open Circulatory Systems

  • A circulatory system consists of a heart and associated vessels.

  • The heart's role is to keep the fluid (blood) moving within the vessels.

  • Circulatory systems may be classified as open or closed.

  • The grasshopper has an open circulatory system.

  • In an open circulatory system, fluid is not always confined to the vessels.

  • A tubular heart pumps hemolymph through a network of channels and cavities in the body.

  • The cavities, or sinuses, contain the animal's organs and are collectively known as the hemocoel.

  • Hemolymph drains back to the heart.

  • The hemolymph of a grasshopper is colorless and does not contain hemoglobin or any other respiratory pigment.

  • Oxygen is taken to cells, and carbon dioxide is removed from them by way of air tubes called tracheae.

  • Tracheae are found throughout the body and provide efficient transport and delivery of respiratory gases while restricting water loss.

Open circulatory system.

Closed Circulatory System

  • All vertebrates and some invertebrates have a closed circulatory system, also known as a cardiovascular system.

  • The system consists of a muscular heart and blood vessels.

  • Blood remains within the blood vessels at all times.

  • Humans have two receiving chambers (atria) and two pumping chambers (ventricles) in the heart.

  • There are three types of vessels: arteries, capillaries, and veins.

  • Arteries carry blood away from the heart, capillaries exchange materials with interstitial fluid, and veins return blood to the heart.

  • Blood is always contained within these vessels and does not run freely into the body unless there is an injury.

  • As blood passes through capillaries, some water is forced out and into the interstitial fluid.

  • Some of this fluid returns directly to a capillary, while some is picked up by lymphatic capillaries.

  • The fluid, now called lymph, is returned to the cardiovascular system by lymphatic vessels.

Closed circulatory systems.

Comparison of Vertebrate Circulatory Pathways

  • Two types of circulatory pathways among vertebrate animals:

    • Fishes have a one-circuit (single-loop) pathway through the body

      • The heart has a single atrium and a single ventricle.

      • The ventricle pumps blood to the gills for gas exchange.

      • Gill capillaries receive O2-poor blood; systemic capillaries receive O2-rich blood.

    • Other vertebrates have a two-circuit (double-loop) circulatory pathway

      • The heart pumps blood to tissues through a systemic circuit.

      • The heart pumps blood to the lungs through the pulmonary circuit.

      • The double pumping action is seen in terrestrial animals that breathe air.

Comparison of circulatory circuits in vertebrates.

  • Amphibians have two atria and a single ventricle.

  • Some mixing of O2-rich and O2-poor blood occurs in amphibians.

  • Most reptiles have a single ventricle with a partial septum, reducing mixing of blood.

  • Crocodilians, birds, and mammals have hearts divided into right and left halves.

  • The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body.

  • The left ventricle is larger than the right ventricle.

  • This arrangement provides adequate blood pressure for both the pulmonary and systemic circuits.

23.2 Transport in Humans

  • The human cardiovascular system pumps blood into blood vessels.

  • Blood vessels take blood to capillaries.

  • Exchanges take place in capillaries.

  • In the lungs, carbon dioxide is exchanged for oxygen.

  • In the tissues, nutrients and oxygen are exchanged for carbon dioxide and other wastes.

  • If the heart stops pumping, death results because exchanges in the lungs and tissues are crucial.

The Human Heart

  • The right side pumps O2-poor blood to the lungs.

  • The left side pumps O2-rich blood to the tissues.

  • The septum separates the right side from the left and prevents the mixing of blood.

  • Each side has two chambers: atria (upper, thin-walled) and ventricles (lower, thick-walled.)

  • Valves are located between the atria and ventricles and between the ventricles and attached vessels.

  • Valves close after blood moves through to keep it moving in the correct direction.

  • Atrioventricular valves are between atria and ventricles, and semilunar valves are between ventricles and attached vessels.

  • Blood path: right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk and arteries → lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → tissues.

  • Pulmonary arteries carry O2-poor blood, and pulmonary veins carry O2-rich blood (opposite of other arteries and veins.)

  • Arteries take blood away from the heart; veins take blood toward the heart.

Heart anatomy and the path of blood throughthe heart.

The Cardiac Cycle

  • The heart's pumping action is called the cardiac cycle.

  • The cardiac cycle consists of the following events:

    • Atrial systole: The atria contract, pumping blood into the ventricles.

    • Ventricular systole: The ventricles contract, pumping blood out of the heart.

    • Diastole: The atria and ventricles relax, filling with blood.

  • The cardiac cycle is controlled by the SA node, a group of cells in the heart that generate electrical impulses.

  • The SA node sends out an electrical impulse every 0.85 seconds, which causes the heart to beat.

  • The heart rate can be affected by factors such as exercise, stress, and medications.

  • An electrocardiogram (ECG) is a test that can be used to measure the electrical activity of the heart.

  • ECGs can be used to diagnose heart problems such as arrhythmias and heart attacks.

  • A defibrillator is a device that can be used to shock the heart back into a normal rhythm.

The cardiac cycle.

Control of the heartbeat.

Blood Vessels

  • Arteries:

    • Transport blood away from the heart.

    • Have a thick wall with a middle layer of smooth muscle and elastic fibers.

    • Elastic fibers allow arteries to expand and accommodate sudden increase in blood volume.

    • Smooth muscle strengthens the wall and prevents overexpansion.

    • Branch into arterioles, which are small arteries that can be regulated by the nervous system.

    • Arterioles can be dilated or constricted to control blood flow and raise blood pressure.

    • Arterioles branch into capillaries, which are extremely narrow tubes that allow exchange of nutrients and waste molecules.

    • Capillary beds are widespread and allow efficient exchange of substances between the blood and interstitial fluid.

    • The entrance to capillary bed is controlled by precapillary sphincters.

    • Blood moves through a shunt from arteriole to venule if capillary bed is closed.

  • Veins:

    • Collect blood from capillary beds and carry it back to the heart.

    • Have thinner walls and wider diameters than arteries.

    • Skeletal muscle contraction pushes blood past valves that prevent backflow.

    • Valves point toward the heart.

    • Blood flows in the direction of reduced pressure, aided by inhalation and chest expansion.

Blood vessels.

Movement of blood in a vein.

The Pulmonary and Systemic Circuits

  • The human cardiovascular system has two major circulatory pathways:

    • Pulmonary circuit

      • Moves blood to and from the lungs

      • Converts O2-poor blood to O2-rich blood

    • Systemic circuit

      • Moves blood to and from other tissues of the body

      • Serves the metabolic needs of the body's cells

  • Figure 23.10 shows the path of blood in both circuits.

23.10 The path of blood.

  • The circulatory system can be divided into two circuits: the pulmonary circuit and the systemic circuit.

  • Pulmonary Circuit:

    • O2-poor blood from all regions of the body collects in the right atrium.

    • Blood passes into the right ventricle.

    • The right ventricle pumps blood to the lungs via the pulmonary trunk and the pulmonary arteries.

    • Carbon dioxide is given off and oxygen is picked up as blood passes through pulmonary capillaries.

    • O2-rich blood returns to the heart via the pulmonary veins.

    • The pulmonary veins enter the left atrium.

  • Systemic Circuit:

    • O2-rich blood enters the left atrium from the lungs and passes into the left ventricle.

    • The left ventricle pumps the blood into the aorta.

    • Arteries branching from the aorta carry blood to all areas and organs of the body.

    • Blood passes through capillaries and collects in veins.

    • Veins converge on the venae cavae, which return the O2-poor blood to the right atrium.

    • Arteries contain O2-rich blood and are bright red; veins contain O2-poor blood and appear dull red or blue when viewed through the skin.

  • Portal System:

    • A portal system begins and ends in capillaries.

    • The hepatic portal vein takes blood from the intestines to the liver.

    • The liver modifies substances absorbed by the intestines and monitors the normal composition of the blood.

    • The hepatic veins carry blood out of the liver into the inferior vena cava.

Lymphatic System

  • Consists of lymphatic vessels and various lymphatic organs.

  • Takes up fat in the form of lipoproteins from the digestive tract and transports it to the circulatory system.

  • Works with the immune system to help defend the body against disease.

  • Takes up excess interstitial fluid and returns it to cardiovascular veins in the shoulders.

  • Most regions of the body are richly supplied with lymphatic capillaries.

  • The construction of the larger lymphatic vessels is similar to that of cardiovascular veins, including the presence of valves.

  • The movement of lymph within these vessels is dependent on skeletal muscle contraction.

  • The lymphatic system is a one-way system that begins at the lymphatic capillaries.

  • The lymphatic capillaries take up excess interstitial fluid, and once it enters the lymphatic vessels, it is called lymph.

  • The lymphatic capillaries join to form larger lymphatic vessels that merge before entering one of two ducts: the thoracic duct or the right lymphatic duct.

  • The thoracic duct is much larger than the right lymphatic duct and serves the lower limbs, abdomen, left arm, and left sides of both the head and the neck.

  • The right lymphatic duct serves the right arm, the right sides of both the head and the neck, and the right thoracic area.

  • The lymphatic ducts enter the subclavian veins.

Lymphatic vessels.

Capillary Exchange in the Tissues

  • Exchange in animals:

    • In some animals, exchange is carried out by each cell individually because there is no cardiovascular system.

    • In animals with a cardiovascular system, interstitial fluid is involved in exchanging materials between capillaries and cells.

    • Amino acids, oxygen, and glucose exit a capillary and enter interstitial fluid to be used by cells.

    • Carbon dioxide and wastes leave interstitial fluid and enter a capillary to be excreted from the body.

    • The cardiovascular system takes blood to the capillaries where exchange occurs.

    • Homeostasis is not maintained without this exchange, and cells perish.

  • Mechanics of capillary exchange:

    • Blood pressure and osmotic pressure are two opposing forces at work along the length of a capillary.

    • Blood pressure is caused by the beating of the heart, while osmotic pressure is due to the salt and protein content of the blood.

    • Blood pressure holds sway at the arterial end of a capillary, and water exits.

    • Blood pressure is reduced by the time blood reaches the venous end of a capillary, and osmotic pressure now causes water to enter.

    • Midway between the arterial and venous ends of a capillary, blood pressure pretty much equals osmotic pressure, and passive diffusion alone causes nutrients to exit and wastes to enter.

    • Diffusion works because interstitial fluid always contains fewer nutrients and more waste than blood does.

  • Lymphatic system:

    • The exchange of water at a capillary is not exact, and excess interstitial fluid is always produced.

    • Excess interstitial fluid is collected by lymphatic capillaries and becomes lymph.

    • Lymph contains all the components of plasma, except it has much less protein.

    • Lymph is returned to the cardiovascular system when the major lymphatic vessels enter the subclavian veins in the shoulder region.

  • Distribution of heat:

    • The blood distributes heat to body parts.

    • When warm, capillaries that serve the skin are open, and the face is flushed.

    • This helps rid the body of excess heat.

    • When cold, skin capillaries close, conserving heat.

Capillary exchange.

23.3 Blood: A Transport Medium

  • Blood is a form of fluid connective tissue

  • Blood has important roles in the body, including:

    • Transport of substances to and from the capillaries

    • Defense of the body against invasion by pathogens

    • Assisting in homeostasis by regulating body temperature and pH

  • Blood in humans has two main portions:

    • Liquid portion called the plasma.

    • Formed elements consisting of various cells and platelets.

  • These portions can be separated by spinning blood in a centrifuge (Fig. 23.14).

Fig 23.14 Components of blood.

Plasma

  • Plasma is mostly composed of water (90-92%) and proteins (7-8%).

  • Plasma also contains smaller quantities of nutrients, wastes, and salts.

  • The salts and proteins in plasma help buffer the blood and maintain a slightly basic pH of 7.4.

  • They also maintain the blood's osmotic pressure, allowing water to enter blood capillaries.

  • Several plasma proteins are involved in blood clotting.

  • Other plasma proteins transport large organic molecules in the blood.

  • Albumin, the most abundant plasma protein, transports bilirubin, a breakdown product of hemoglobin.

  • Lipoproteins transport cholesterol.

Formed Elements

  • The formed elements in blood are red blood cells, white blood cells, and platelets.

  • Red blood cells, also known as erythrocytes, transport oxygen.

  • Red blood cells lack a nucleus and contain hemoglobin, a respiratory pigment.

  • There are 6 million red blood cells in a small drop of whole blood, each containing about 250 million hemoglobin molecules.

  • Hemoglobin contains iron, which loosely combines with oxygen to transport it.

  • Insufficient red blood cells or hemoglobin causes anemia and fatigue.

  • High altitudes stimulate red blood cell formation.

  • Red blood cells are continuously manufactured in certain bones, stimulated by the hormone erythropoietin (EPO).

  • EPO is produced by the kidneys and liver, and is available as a drug to treat anemia.

  • EPO has been abused by athletes to enhance performance.

  • Red blood cells synthesize hemoglobin and lose their nuclei before being released from the bone marrow into the blood.

  • After living for about 120 days, red blood cells are destroyed, mainly in the liver and spleen, where they are engulfed by large, phagocytic cells.

  • Hemoglobin is released when red blood cells are destroyed. The iron is recovered and returned to the red bone marrow for reuse.

  • Another portion of the molecules (i.e., heme) undergoes chemical degradation and is excreted by the liver as bile pigments in the bile. The bile pigments are primarily responsible for the color of feces.

  • White blood cells, also called leukocytes, help fight infections. They are usually larger than red blood cells, have a nucleus, lack hemoglobin, and appear translucent if unstained.

  • There are approximately 5,000–11,000 white blood cells in a small drop of whole blood.

  • Growth factors are available to increase the production of all white blood cells, and these are helpful to people with low immunity, such as AIDS patients.

  • Red blood cells are confined to the blood, but white blood cells are able to squeeze between the cells of a capillary wall. Therefore, they are found in interstitial fluid, lymph, and lymphatic organs.

  • When an infection is present, white blood cells greatly increase in number. Many white blood cells live only a few days, while others live months or even years.

  • Neutrophils are white blood cells that squeeze through the capillary wall and enter the interstitial fluid, where they phagocytize foreign material. Monocytes come on the scene next and are transformed into macrophages—large, phagocytizing cells that release white blood cell growth factors.

  • Lymphocytes, another type of white blood cell, play an important role in fighting infection. T cells attack the body’s cells that are infected with viruses, while B cells produce antibodies to protect the body against certain types of antigens.

  • An antigen is most often a protein but sometimes a polysaccharide. Antigens are present in the outer covering of parasites or in their toxins. When antibodies combine with antigens, the complex is often phagocytized by a macrophage.

Formed elements.

Platelets and Blood Clotting

  • Platelets are fragments of megakaryocytes in the red bone marrow

  • 200 billion platelets are produced daily

  • A small drop of whole blood contains 150,000 to 300,000 platelets

  • Platelets are involved in blood clotting or coagulation

  • Hemophilia is an inherited clotting disorder caused by the liver's inability to produce one of the clotting factors

  • Prothrombin and fibrinogen are two proteins involved in blood clotting that are manufactured and deposited in blood by the liver

  • Vitamin K is necessary for the production of prothrombin and can be found in green vegetables and formed by intestinal bacteria

  • Platelets clump at the site of a damaged blood vessel and form a plug that temporarily seals the leak

  • Prothrombin activator converts prothrombin to thrombin, which requires calcium ions

  • Thrombin severs two short amino acid chains from each fibrinogen molecule, forming long threads of fibrin that wind around the platelet plug and provide the framework for the clot

  • Red blood cells are trapped within the fibrin threads, making the clot appear red

  • Clot retraction follows, during which the clot gets smaller as platelets contract and serum is squeezed from the clot

  • A fibrin clot is present only temporarily, and plasmin destroys the fibrin network and restores the fluidity of the plasma as soon as blood vessel repair is initiated.

Blood clotting.

Cardiovascular Disorders

  • Cardiovascular disease (CVD) is the leading cause of untimely death in Western countries.

  • In the United States, about 31% of the population suffers from hypertension, which is high blood pressure.

  • Normal blood pressure is 120/80 mm Hg.

  • Hypertension occurs when blood pressure readings are higher than these numbers.

  • Hypertension is sometimes called a silent killer because it may not be detected until a stroke or heart attack occurs.

  • Heredity and lifestyle contribute to hypertension.

  • Hypertension is often seen in individuals who have atherosclerosis, an accumulation of soft masses of fatty materials, particularly cholesterol, beneath the inner linings of arteries.

  • Atherosclerosis begins in early adulthood and develops progressively through middle age.

  • To prevent its onset and development, a diet low in saturated fat and rich in fruits and vegetables is recommended.

  • Smoking, alcohol or other drug abuse, obesity, and lack of exercise contribute to the risk of atherosclerosis.

  • Plaque can cause a clot to form on the irregular arterial wall.

  • If thromboembolism is not treated, serious health problems can result.

  • A cerebrovascular accident, also called a stroke, often occurs when a small cranial arteriole bursts or is blocked by an embolus.

  • If a coronary artery becomes completely blocked due to thromboembolism, a heart attack can result.

  • Two surgical procedures are frequently performed to correct a blockage or facilitate blood flow: coronary bypass operation and balloon angioplasty.

Atherosclerosis.

Treatment for blocked coronary arteries.